1. Field of the Invention
The invention relates to a filter for collecting mercury vapour or minute mercury droplets carried in a gas as a relatively low concentration, and the method of filtering the mercury from the gas stream.
2. Prior Art
Mercury has been known to be a toxic substance for many years, but has been difficult to collect in the vapour phase, that is as commonly found above a sample of metallic mercury at normal temperature and pressure. For convenience, in this specification, mercury in this phase is referred to as "mercury vapour" and could include, in some instances, discrete mercury droplets of very small size as might be found in industrial applications where mercury is handled.
A common source of mercury contamination is in dentists' offices where mercury vapour is frequently present during the preparation of amalgams for dental fillings. It can also occur in hospitals or laboratories when mercury thermometers have been broken and the mercury spilled. Because the concentration of mercury in these particular environments is relatively low, to the inventor's knowledge, very little work has been done in developing compact, low cost mercury filters to handle the low concentrations of these environments.
Mercury filters to use in environments similar to the present invention are disclosed in U.S. Pat. Nos. 3,956,458 issued to Anderson in 1976 and 4,139,354 issued to Giles in 1979. These patents both use a filter element which includes iodine impregnated charcoal particles encapsulated within a cavity through which air can pass. The first device also uses a sulphur filter and an electrostatic precipitator which complicates the structure considerably particularly due to high voltage requirements. These devices utilize the absorption power of iodine impregnated charcoal and do not utilize amalgamating properites of mercury as in the present invention. The present inventor considers that the filters disclosed in these two patents would have relatively short operating lives when compared with the filter of the present invention, ie. the filters would likely saturate more quickly, and, for a particular physical size of filter, would absorb mercury at a slower rate than the present invention. Also, for an effectiveness equal to the present invention, such filters would require an excessive amount of charcoal which would correspondingly require a larger, noisier fan than the present invention.
The present invention is particularly adapted for absorbing low concentrations of mercury in an air flow of low speed such as found in the above environments, and as such would require only a low powered fan. Clearly, with a suitable increase in size the invention could be used in higher concentrations contained in greater air flows as found in heavy duty industrial applications. To the inventor's knowledge, in industrial applications, mercury compounds only, but not mercury itself, are trapped in conventional electrostatic precipitations which are also used to trap other air borne contaminants. Conventional electrostatic precipitators usually include a stack of spaced plates or members, some of which are charged to a relatively high potential, for example 10 to 15 kilovolts. The precipitators commonly include means to vibrate the plates to collect contaminants collected thereon. These industrial filters are relatively complex structure which clearly would be relatively expensive and inappropriate for use in low concentration, small flow applications.